Imagine Wisconsin about 500 million years ago. Located near the equator,
the area was a tropical paradise, full of barrier islands, lagoons and
sandy, shallow embayments, similar to the coast of Florida today. Giant
jellyfish cruised the warm waters.

This environment, says Whitey Hagadorn of the California Institute of
Technology, was the perfect setting for some large jellyfish to leave their
legacy. He says the Cambrian sediment trapped hordes of jellyfish, some
up to a meter across, preserving their shape and form. In the February
issue of Geology, Hagadorn, Robert Dott and Dan Damrow describe
how they think these jellyfish remained so well preserved on Wisconsin's
Upper Cambrian shoreline.

"These are the
largest jellyfish in the fossil record and it's one of only two jellyfish
deposits in the entire fossil record which is rather ironic because they're
both upper Cambrian in age," Hagadorn says. The other fossil site is in
New Brunswick.

What makes these fossils unusual, Hagadorn excitedly explains, is both
their size and abundance, as well as their exceptional preservation in
a medium- to coarse-grained sand. "You never get soft bodied preservation
in that kind of coarse grain size" in the Cambrian, he says.

A fossil slab from Upper
Cambrian Mount Simon-Wonewoc Sandstone in central Wisconsin, believed to
be the impression of a jellyfish. Each scale bar is 10 centimenters (4
inches). Courtesy of Whitey Hagadorn.

Hagadorn and his colleagues looked at present-day shores to help them
identify the factors that led to the exceptional preservation of these
fossils. Firstly, Hagadorn says, in the Upper Cambrian, few scavengers
were around eating the jellyfish carcasses. "If jellyfish are washed up
on the beach today, they are very likely going to be scavenged by terrestrial
organisms." Similarly, few animals were turning up sediments. "Think about
going to a modern beach today and seeing sand crabs burrowing through the
sand at the shoreline."

And lastly, Hagadorn says, the jellyfish had to have remained underneath
a large volume of sediment. Jellyfish could swim into the sandy embayments
to migrate, prey on other organisms or reproduce. And, a strong storm event
could have then trapped the jellyfish. Hagadorn and his colleagues found
at least seven different layers of jellyfish fossils over a couple of meters
of sediment. Because there is no unconformity between the layers, Hagadorn
says, the stranding events could represent a period of time anywhere from
one season to a hundred thousand years.

Hagadorn hopes to learn more about the Cambrian marine community, and
to better understand how soft-bodied organisms fossilize. "When people
find a T-rex, that doesn't excite me that much, because a T-rex
has bones and teeth --really easy to fossilize," Hagadorn says. "But to
preserve a jellyfish, that's hard, because it has no hard parts. Something
is there we don't understand."

The Upper Cambrian Mount Simon-Wonewoc Sandstone in central Wisconsin
is well-known for the preservation of the tire-track-sized trails Climactichnites.
About four years ago, Damrow, a commercial fossil dealer, was collecting
these trace fossils and decided to send Hagadorn some photographs from
the fossil deposit. Several of the photographs had large disc-shaped structures
on them that looked like medusae impressions. And that piqued Hagadorn's
curiosity.

At that point, Hagadorn says, they could not idenitfy the fossils. "You
can get a lot of disc-shaped structures produced by other processes. There
are a million ways to produce a round mark on a rock," he says. But after
meeting up with Bob Dott in Wisconsin and analyzing the fossils at the
site and at Caltech, they concluded the fossils were indeed medusae, or
jellyfish. "There's just no other way to produce these structures unless
you have a soft-bodied spheroidal bag, or jellyfish stranded on a shoreline."

But, some paleontologists remain skeptical. Simon Conway Morris, a paleontologist
at the University of Cambridge who studies the Cambrian explosion, says
that while the fossils look like jellyfish, other possibilities remain,
including a connection to the Climactichnites, which he describes
as "extremely spectacular trace fossils made by something akin to a gigantic
slug." The fossils, he says, could even be some sort of egg case. That
possibility is unlikely, Morris says, but everything related to the fossils
is still speculative.

The authors address the possible Climactichnites connection in
their paper, however, "their explanation is as good as anybody can manage
at the moment. It's a very sort of curious find," Morris says. "As exciting
as the results are, they do actually pose additional questions which are
clearly going to open new lines of inquiry." And that Morris says, is what
science is all about.